Composition and method for treating a subterranean formation

ABSTRACT

Aqueous liquid compositions for increasing the permeability of a subterranean formation are disclosed, the compositions being characterized by provision of fluoborate anion (fluoboric acid) and a specified compound or compounds, or mixture thereof, which chelate aluminum ions and aluminum fluoride species. Methods of treating a subterranean formation by injection of the composition(s) into the formation are also described.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 09/311,690 filed May 13, 1999 now abandoned.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention relates to compositions and methods for increasing thepermeability of a subterranean formation. More particularly, theinvention relates to acidizing compositions and methods for theirapplication in subterranean formations comprising siliceous material.

B. Background

Acidizing of siliceous formations by injection of compositions referredto as mud acid is common practice in oilfield operations. As commonlyunderstood, the expression “mud acid” refers to an aqueous base mixtureformed by blending hydrofluoric acid (HF) and at least one ofhydrochloric acid (HCl), acetic acid (C₂H₄O₂), or formic acid (CH₂O₂),the most common mixture employed being formed from HF and HCl. Often, ifacetic acid or formic acid are the acids combined with the HF, the mudacid is referred to as “organic mud acid”. As is well recognized in theart, the origin of the species in the acidizing solution or mixture isnot critical, so that a “mud acid”, which might be formed by directlyblending, e.g., HF and HCl, also is understood to include aqueousmixtures or solutions formed by mixing components which quickly react toform the desired ionic species in the solution or mixture. The ratiosand amounts of the acids combined may vary over wide ranges, with thelower limits being more a matter of practicality rather thanoperability, and the upper limits being a matter of mutual solubility ofthe acids. Most typically, a mud acid is formed by combining about 3 toabout 25 percent HCl and about 1 to about 10 percent HF, bothpercentages by weight, in aqueous solution, and is typicallysubstantially free of other acidic species. As will be recognized bythose skilled in the art, mud acids may also contain, and commonly do,one or more functional additives, such as inhibitors, diverting agents,and/or surfactants.

Although conventional treatments of siliceous clay containing formationswith mud acids have generally proven effective for a short time, theimprovements in production are frequently short lived. One explanationfor this phenomenon is that the mud acid reacts rapidly with thesubterranean formation in the vicinity of or near wellbore area, usuallythe first few inches around the wellbore, thus spending so rapidly thatpenetration deep into the subterranean formation is not achieved.Subsequently, fines in the subterranean formation migrate into theacidized near wellbore area and replug the area.

One solution to this problem is that taught in U.S. Pat. No. 3,828,854(Templeton et al) and in the “Introduction” section of Society ofPetroleum Engineers Paper No. 5153. The approach taken is the provision,down the wellbore, of a composition or solution which generates HFslowly, so that the solution is placed in contact with the subterraneanformation before a significant amount of the HF is generated. Thecomposition is a relatively high pH aqueous solution of a water solublefluoride salt and at least one water reactive organic acid ester.

U.S. Pat. No. 2,300,393 (Ayers, Jr.) discloses treatment of subterraneanformations with fluoboric acid, optionally containing small amounts ofHF. Ayers, Jr. also teaches that the fluoboric acid treatment may befollowed by HCl containing an inappreciable amount of hydrofluoric acid,or optionally, by a mixture of HCl and fluoboric acids. Again, U.S. Pat.No. 2,425,415 (Bond et al.) describes an acidizing procedure in whichthe subterranean formation is first contacted with a fluoboric acidsolution which does not contain free HF, but which contains an excess ofboric acid, followed by contact of the subterranean formation withaqueous fluoboric acid containing excess HF. U.S. Pat. No. 2,663,689(Kingston el. al.) describes the use of boric acid in aqueous HCl—HF toavoid precipitation of insoluble fluoride salts and fluorosilicic acid.U.S. Pat. No. 4,151,878 (Thomas) is directed to the use of aconventional mud acidizing solution (HCl—HF), followed by fluoboric acidsolution. The use of fluoboric acid as an overflush is believed to deterclay migration and thereby significantly reduce or delay productiondecline which is often otherwise encountered shortly after conventionalmud acidizing treatments.

The Thomas patent also describes injection of a fluoboric acid solution,followed by mud acid (HCl—HF) solution. According to the patent, thetechnique may be used in formations which have a tendency to pluginitially upon contact with mud acid, or with HCl commonly used as apreflush ahead of mud acid. When contacted initially with fluoboricacid, such subterranean formations show little or no plugging effectswhen subsequently treated with mud acid.

However, the Thomas patent does not specifically address formationscontaining zeolites and chlorites. As will be recognized by thoseskilled in the art, the use of traditional mud acid is not advisable insubterranean formations which comprise or contain HCl-sensitivematerials, e.g., zeolite and chlorites. Additionally, fluoride in themud acid is believed to bind with aluminum in the subterranean formationand promote deposition of hydrated silica, thereby causing plugging. Forexample, severe, damaging precipitation of aluminum fluorides during theHF reactions was discovered with formic-HF and acetic-HF fluid systems.See, C. E. Shuchart, et al., “Improved Success in Acid Stimulations witha New Organic-HF System,” SPE 36907 presented at 1996 European PetroleumConference, Milan, Italy. To overcome this problem, Rogers et al.disclosed the use of citric acid as a chelating agent for aluminum toprevent such deposition or formation of hydrated silica gel. The optimumtreatment formulation identified therein consisted of 10 percent citricacid and 1.5 percent HF acid, with no additives except corrosioninhibitor. One important disadvantage of this particular method is thatthe use of hydrofluoric acid primarily addresses damage or scaling inthe initial few inches of the subterranean formation around thewellbore, as previously indicated.

Accordingly, there has been a need to extend acidization or stimulationtreatment to deeper depths in the formation, e.g., up to a 3 to 5 feetradius from the wellbore, to avoid a rapid decline in production bystabilizing fines and precipitation of acidization products near thewellbore. The invention addresses this need.

C. SUMMARY OF THE INVENTION

The present invention relates to novel acidic compositions useful intreating a well, and to methods for increasing the permeability of asubterranean formation utilizing the compositions as preflush, main, orpostflush treatments for the formation. In one embodiment, the inventioncomprises an aqueous acidic solution or mixture formed by blending anaqueous liquid; fluoboric acid; and an acid, or mixture of acids, whichsequester or chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof.In a further embodiment, the composition comprises an aqueous acidicsolution or mixture formed or produced by blending an aqueous liquid; afluoride ion source, as defined, or HF; a boron source; and an acid, ormixture of acids, which chelate aluminum ions and aluminum fluoridespecies, or an ammonium or potassium salt or salts of such acids, or amixture thereof. As utilized herein, the expression “fluoride ionsource” is taken as referring to a compound or compounds, other than HF,or aqueous solutions of the compound or compounds, that will providefluoride ion or ions in an aqueous liquid. Similarly, the term “boronsource” is considered to define a compound or compounds, or an aqueoussolution of the compound or compounds, providing boron ions orboron-containing anions which are reactive with an aqueous liquid or acomponent in the aqueous liquid to form the BF₄ ⁻ anion in the aqueousliquid.

In a principal embodiment, therefore, the invention relates to acomposition useful for treating a subterranean formation comprising anaqueous acidic solution or mixture formed by blending an aqueous liquid;a fluoride ion source; a boron source; and an acid, or mixture of acids,which chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof,and to a method of treating a subterranean formation utilizing thecomposition. In a preferred aspect of this embodiment, the fluoride ionsource is selected from ammonium bifluoride and ammonium fluoride, andmixtures thereof, the boron source is boric acid, and the acid whichchelates aluminum ions and aluminum fluoride species is selected frompolycarboxylic acids, polyaminopolycarboxylic acids, andmonoaminopolycarboxylic acids. In one very preferred aspect of thisembodiment, the fluoride ion source is ammonium bifluoride, the boronsource is boric acid, and the acid which chelates aluminum ions andaluminum fluoride species is selected from citric acid, malic acid,2-hydroxyethyliminodiacetic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, and mixtures thereof.

In another embodiment of the invention, the composition is formed byblending an aqueous liquid; HCl in specified amount; a fluoride ionsource; a boron source; and an acid, or mixture of acids, which chelatealuminum ions and aluminum fluoride species, or an ammonium or potassiumsalt or salts of such acids, or a mixture thereof, and to a method oftreating a subterranean formation utilizing the composition. Preferably,the fluoride ion source is selected from ammonium bifluoride andammonium fluoride, and mixtures thereof, the boron ion source is boricacid, and the acid which chelates aluminum ions and aluminum fluoridespecies is selected from polycarboxylic acids, polyaminopolycarboxylicacids, and monoaminopolycarboxylic acids. In one very preferred aspect,the fluoride ion source is ammonium bifluoride, the boron ion source isboric acid, and the acid which chelates aluminum ions or aluminumfluoride species is selected from citric acid, malic acid,2-hydroxyethyliminodiacetic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, and mixtures thereof.

In a further embodiment, the invention relates to a composition usefulfor treating a subterranean formation comprising an aqueous acidicsolution or mixture formed by blending an aqueous liquid; HF; a boronsource; and a compound or compounds which chelate aluminum ions andaluminum fluoride species, and to a method of treating a subterraneanformation utilizing the composition. Preferably, the boron source isboric acid, and the compound(s) which chelate aluminum ions or aluminumfluoride species are selected from polycarboxylic acids,polyaminopolycarboxylic acids, and monoaminopolycarboxylic acids. In onevery preferred aspect of this embodiment, the boron source is boricacid, and the compound which chelates aluminum ions or aluminum fluoridespecies is selected from citric acid, malic acid,2-hydroxyethyliminodiacetic acidN-(2-hydroxyethyl)ethylenediaminetriacetic acid and mixtures thereof.

In their most preferred aspects, the acid treatment compositions of theinvention also include non-interfering ionic species in a concentrationor concentrations in the aqueous mixture effective to provide a level,or increase the ionic strength of the composition to a level, sufficientto inhibit migration of clay particles in a subterranean formation whenthe aqueous mixture is applied to or injected into the subterraneanformation. The non-interfering ionic species may be derived fromprecursor compositions employed in preparing the compositions of theinvention, or may be provided by addition of a non-interfering solublesalt or salts during formulation of the compositions of the invention.As understood herein, the term “non-interfering”, in referring to theionic species, simply indicates that the ionic species do not interfereto any significant extent with the formulation of, or desired treatmentfunction(s) of the compositions of the invention, while the term“soluble” indicates that any compound or compounds added, ornon-interfering species present in the aqueous mixtures, have sufficientsolubility in the aqueous mixture to provide the desired concentrationlevel. Preferably, the total ionic strength of the compositions of theinvention will range from 2 percent to 10 percent, most preferably from3 percent to 7 percent, all percentages by weight. Thus, in the casewhere the invention comprises an aqueous solution or mixture formed byblending an aqueous liquid; fluoboric acid; and an acid, or mixture ofacids, which chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof,ionic species, such as may be provided by addition of at least onenon-interfering soluble salt, e.g., NH₄Cl, or KCl, may be present in aconcentration effective to provide an ionic strength of the inventioncomposition sufficient to inhibit clay particle migration. Where theinvention composition is formed by blending an aqueous liquid; afluoride ion source; a boron source; and an acid, or mixture of acids,which chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof,the concentration of non-interfering soluble ionic species may bederived sufficiently from the compositions or precursors utilized toformulate the compositions of the invention, or appropriate salt(s) maybe added in a concentration effective to increase the ionic strength ofthe composition to a level sufficient to inhibit clay particlemigration. In the case where the composition is formed by blending anaqueous liquid; HCl in specified amount; a fluoride ion source; a boronsource and an acid, or mixture of acids, which chelate aluminum ions andaluminum fluoride species, or an ammonium or potassium salt or salts ofsuch acids, or a mixture thereof, non-interfering soluble salt speciesmay be provided or added to the mixture in a concentration effective toinsure that the ionic strength of the composition is at a levelsufficient to inhibit clay particle migration. This will also be thecase where the compositions of the invention are formed by blending anaqueous liquid; HF; a boron source; and an acid, or mixture of acids,which chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof.

An important aspect of the invention is the requirement of low pH forthe compositions of and method of the invention, i.e., the compositionsare acidic and are employed as such. The compositions will thus beblended with at least the components specified, but may also beformulated by further addition of an amount of a non-interfering acid,or acids, sufficient to insure pH levels in the acid range, preferablystrong acid range. Low pH is considered to aid in maintaining Si and Sispecies in solution.

As indicated, the invention further includes the use of each compositiondisclosed in a method for treating a subterranean formation to increasethe permeability thereof. The terms “treating” or “treatment” are takenherein, as indicated, to include preflush, main or acidizing, orpostflush treatments, permeability increase being achieved by reactionwith or dissolution of components of the formation. In treating asubterranean formation, the compositions are commonly injected into theformation at a pressure referred to as matrix pressure and allowed toreact with or dissolve the minerals or components composing theformation, in amount sufficient or effective to increase thepermeability of the formation.

D. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates sequential acid spending testing designed to simulateacid solution penetrating a subterranean formation.

FIG. 2 is a graph illustrating silicon ion concentration in sequentialspending effluents at 77° C.

FIG. 3 is a graph illustrating fluoride ion concentration in sequentialspending effluents 77° C.

FIG. 4 is a graph illustrating silicon ion concentration in sequentialspending effluents at 93° C.

FIG. 5 is a graph illustrating fluoride ion concentration in sequentialspending effluents at 93° C.

FIG. 6 is a graph illustrating core flow results showing damage removalby compositions and treatment according to the invention at 93° C.

FIG. 7 is a graph illustrating weight loss over time by 2.5 g samples ofsolids contacted with conventional acids and a treatment composition ofthe invention.

FIG. 8 is a graph illustrating weight loss over time by 5.0 g samples ofsolids contacted with conventional acids and a treatment composition ofthe invention.

FIG. 9 is a graph illustrating weight loss over time by 10.0 g samplesof solids contacted with conventional acids and a treatment compositionof the invention.

FIG. 10 is a graph illustrating weight loss of slides after contact withvarious HF containing acids, including two acids according to theinvention.

FIG. 11 is a graph illustrating results of permeability tests describedhereinafter in example 9.

FIG. 12 is a graph illustrating results of permeability tests describedhereinafter in example 10.

FIG. 13 is a graph illustrating results of permeability tests describedhereinafter in example 11.

FIG. 14 is a graph illustrating results of permeability tests describedhereinafter in example 12.

FIG. 15 is a graph illustrating results of permeability tests describedhereinafter in example 13.

E. DETAILED DESCRIPTION OF THE INVENTION

In general, as indicated, compositions according to the invention may beprepared by mixing the required components in an aqueous liquid. Theexpression “aqueous liquid” is understood as including a wide spectrumof water-based liquids, including, but not limited to, fresh water, seawater, dilute acids, and brines, so long as any components of theaqueous liquid do not interfere significantly with the formation of orperformance of the compositions of the invention. Additionally, as alsoindicated, one or more of the precursor compounds or compositions mayfirst be blended with or dissolved in an aqueous liquid, if desired,before blending with aqueous liquid and one or more components to formthe compositions of the invention. As will be recognized by thoseskilled in the art, the aqueous liquid of the invention may containadditives, inhibitors, etc., as are common in formation treatmentprocedures.

The sequence of blending the components of the aqueous acidic mixture ofthe invention is not critical, i.e., the components or aqueous solutionsthereof may be blended in any desired order or sequence. Preferably,however, in the embodiments of the invention where a boron source is tobe blended in the aqueous liquid, and where the boron source is oflimited solubility, the acid, etc. chelant for aluminum and aluminumfluoride species and the fluoride ion source, or HF, are blended firstwith the aqueous liquid, followed by the blending or addition of theboron source. For example, the desired amounts of citric acid orN-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) and ammoniumbifluoride may be mixed with fresh water in a mixing vessel untildissolved. Thereafter, a boron source, such as boric acid, may be addedto the vessel. The boric acid is preferably added last since it does noteasily dissolve in fresh water, but will readily be taken up by theacid-containing solution. Although the compositions may be blendedoffsite, they will normally be blended at the surface proximate the wellsite, or on the fly, and pumped downwell to the site selected fortreatment, which may be isolated by suitable means, as is known in theart. Alternatively, they may be blended as concentrates, and thendiluted at the well site, either on the surface, or on the fly.Compositions or solutions according to the invention may be used attemperatures ranging from about 20° C. to about 170° C.

As will be understood by those skilled in the art, blending of thecomponents or compounds specified herein in aqueous liquid gives rise tochemical reactions in the aqueous liquid, to the effect that, in eachembodiment, a complex mixture of ionic species is produced in theaqueous liquid. Exemplary formulation reaction equations, which, in thecase of equation (1), illustrates the equilibrium reaction forhydrolysis of fluoboric acid, are shown, as follows:

As shown by the reverse arrows, and as will be understood by thoseskilled in the art, the reactions will reach equilibrium, so that, forexample, as shown by equation (4), a small concentration of HF will bepresent in the aqueous mixture. In the second reaction shown, citricacid, while also used herein as a chelating agent for aluminum andaluminum fluoride species, provides the hydrogen ions for the productionof HF.

A key feature of the invention is, of course, the injection of thecompositions described into the subterranean formation and penetrationthereof to a greater depth before spending. This is believedaccomplished by the presence of the BF₄ ⁻ (fluoborate) species in theaqueous liquid, which may be said to generate acid species slowly “insitu”, i.e., at or in the subterranean formation, which attack thesilica, silicates, and aluminosilicates in the formation. Inconventional terminology, and with reference to the equations provided,fluoboric acid in the aqueous liquid is considered to hydrolize slowly,forming HF in the liquid, which attacks silica in the formation. As theHF is spent in acidizing the formation, the equilibrium will be shiftedto the right in the reaction illustrated in equation (1) in order togenerate more HF to replace that spent.

As will be appreciated by those skilled in the art, and with referenceto equations 2 through 4, the blending of, in aqueous liquid orsolution, at or by achievement of appropriate pH, a fluoride ion source,a boron source, and an acid, or mixture of acids, which chelate aluminumions and aluminum fluoride species, or an ammonium or potassium salt orsalts of such acids, or a mixture thereof, will produce the fluoborateanion (fluoboric acid) in the aqueous liquid. Additionally, HCl, afluoride ion source, a boron source, and an acid, or mixture of acids,which chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof,will also produce the fluoborate anion (fluoboric acid) in the aqueousliquid. Similarly, HF and a boron source will also produce thefluoborate anion (fluoboric acid) in the aqueous liquid. Accordingly,the various embodiments of the invention are linked in that all requirean aqueous acidic liquid containing or comprising fluoborate anion(fluoboric acid), no matter how supplied, and also formed with an acid,or mixture of acids, which chelate aluminum ions and aluminum fluoridespecies, or an ammonium or potassium salt or salts of such acids, or amixture thereof.

Within the limits of practical operation, the concentration of thefluoborate anion (fluoboric acid) in the aqueous liquid is not critical.In the embodiments of the invention wherein the BF₄ ⁻ anion (fluoboricacid) is prepared or formed in the aqueous liquid, the concentrations ofthe components or reactants blended in the aqueous liquid will beapportioned in amounts effective to obtain or provide the desiredconcentration of the BF₄ ⁻ anion (fluoboric acid) in the aqueous liquid.For example, wherein the fluoboric acid is formed by reaction of afluoride ion source, such as ammonium bifluoride, a boron source, suchas boric acid, and an acid, or mixture of acids, which chelate aluminumions and aluminum fluoride species, or an ammonium or potassium salt orsalts of such acids, or a mixture thereof, such as citric acid orN-(2-hydroxyethyl)ethylenediaminetriacetic acid, the fluoride ionsource, the boron source, and an acid, or mixture of acids, whichchelate aluminum ions and aluminum fluoride species, or an ammonium orpotassium salt or salts of such acids, or a mixture thereof, arepreferably, though not necessarily, blended in stoichiometric orapproximately stoichiometric amounts or concentrations. In the aspect ofthe invention wherein HCl, a fluoride ion source, a boron source, and anacid, or mixture of acids, which chelate aluminum ions and aluminumfluoride species, or an ammonium or potassium salt or salts of suchacids, or a mixture thereof, are provided or blended in aqueous liquid,the HCl and the fluoride ion source are preferably provided inapproximately stoichiometric amounts, and an acid, or mixture of acids,which chelate aluminum ions and aluminum fluoride species, or anammonium or potassium salt or salts of such acids, or a mixture thereof,may be provided in lesser amounts, if desired. Again, in the embodimentof the invention utilizing HF, the HF and boron source may be suppliedin stoichiometric or approximately stoichiometric amounts, and thecompound or compounds which chelate aluminum ion and aluminum fluoridespecies, need not be in such concentration. Accordingly, as will beunderstood by those skilled in the art, the ratios and concentrations ofthe components or reactants may be varied extensively, so long as theaqueous liquid contains an amount or concentration of the BF₄ ⁻ anion(fluoboric acid), which, when supplied in sufficient volume or amount inor to the subterranean formation, is effective to increase thepermeability of a subterranean formation. Generally, the concentrationsof fluoboric acid blended with or formed in the aqueous liquid employedare those effective to achieve an observable improvement instabilization of the clays and fines in the remote areas of theformation. Such a stabilizing effect can be recognized by improvedproduction over a more prolonged period of time than would have beenpredicted based on previous experience in that field, or, for example,by laboratory techniques such as core flow tests or by examination of aformation sample using a scanning electron microscope as discussed inSociety of Petroleum Engineers Paper No. 6007. Preferably, treatmentcompositions, once the starting ingredients have been mixed anddissolved in aqueous liquid or water, which contain from about 1 weightpercent or less up to about 20 weight percent BF₄ ⁻, basis HBF₄, may beemployed. More preferably, the treatment composition comprises fromabout 2 to about 10 weight percent BF₄ ⁻, basis HBF₄. Unless otherwisespecified, or evident from context, all component percentages expressedhereinafter are by weight, based on the total weight of the componentand the rest of the mixture.

In the embodiment wherein HF and a boron source are used to formfluoboric acid in the aqueous liquid, the relative concentrations orratios of boron source, e.g., boric acid, and hydrofluoric acid used inpreparing the mixture can be adjusted to tie up all of the freehydrofluoric acid (e.g. as the reaction product of fluoboric acid) or toleave some excess hydrofluoric acid (e.g. unreacted with the boronsource). By adjusting the relative amounts of hydrofluoric acid to boronsource in the mixture, fine tuning of the amount of free hydrofluoricacid in the composition of the invention may be achieved. Where excessHF is present, the amount of excess HF will preferably be less thanabout 1 percent.

In general, the components used in formulating the compositions of theinvention are known and may be obtained from commercial chemicalsources, or they may be prepared by well known procedures. For example,fluoboric acid, HF, HCl, various fluoride ion sources, such as ammoniumbifluoride, various acids or ammonium or potassium salts which chelatealuminum or aluminum fluoride species, such as citric acid andN-(2-hydroxyethyl)ethylenediaminetriacetic acid, and various boronsources, such as boric acid, may be obtained readily. Commercial gradecomponents may be utilized, of standard strengths available, so long asany extraneous species present therewith do not interfere significantlywith the formulation of or function of the compositions of theinvention. As used herein, the expression “aluminum fluoride species”refers to aluminum and fluorine-containing anions formed by reactions ofone or more components of the aqueous liquid with components of thesubterranean formation. Assuming that the principal reaction is, asconventionally understood, with HF from the fluoboric acid in theaqueous fluid, the reaction equation is shown, as follows:HF(hydrofluoric acid)+Al₂Si₄O₁₆(OH)₂(clay)→H₂SiF₆(fluosilicicacid)+AlF_(x) ^((3-x))(aluminum fluoride )+H₂O(water)  (5)

-   -   wherein x is a number from 1 to 6.        Actually, this is believed to be only the initial stage of a        complex reaction sequence. Depending on the free fluoride        concentration, aluminum fluorides are believed present as Al³⁺,        AlF²⁺, AlF₂ ⁺, AlF₃, AlF₄ ⁻, AlF₅ ²⁻, and AlF63⁻. Silicon        fluorides may exist as SiF₄, SiF₅ ¹⁻, and SiF₆ ²⁻.

The silicon fluorides and more-fluoride-rich aluminum species arebelieved to react with additional clay, extracting aluminum and perhapsprecipitating hydrated silica. For example, fluosilicic acid may reactwith additional clay to yield a hydrated silica, i.e., silica gel, asoluble aluminum fluoride species and other byproducts as follows:H₂SiF₆+Al₂Si₄O₁₆(OH)₂→H₂SiO₃+AlF²⁺+Si(OH)₄  (6)The reaction of equation (5) is referred to as the primary reaction andthe reaction of equation (6) as the secondary reaction. Silicaprecipitation may occur according to equation (6).

Importantly, several embodiments of the invention composition furtherinclude an effective amount of an acid, or mixture of acids, whichchelate aluminum ions and aluminum fluoride species, or an ammonium orpotassium salt or salts of such acids, or a mixture thereof. As alsoindicated, in the case where HF is employed in formulating the inventioncomposition, a greater variety of sequestering compounds may beemployed. While not wishing to be bound by any theory of invention, itis believed that the sequestering of the aluminum and/or aluminumfluoride species by the acid(s), or ammonium or potassium salt(s)thereof, or sequestering compound or compounds, frees fluoride ions toassociate in solution with Si ions and maintain the solubility thereof.

Preferably, any acid, or ammonium or potassium salt thereof, whichsequesters or chelates aluminum, or aluminum fluoride species, may beemployed in formulating the compositions of the invention. Theexpression “acid, or mixture of acids, which chelate aluminum ions andaluminum fluoride species, or an ammonium or potassium salt or salts ofsuch acids, or a mixture thereof” is understood as including an acid ofthe type described, a mixture of such acids, the ammonium salt of and amixture of ammonium salts of such acids, the potassium salt of and amixture of potassium salts of such acids, and any mixture of such acidsand ammonium or potassium salts of such acids. Where mixtures of theacid, acids, salt, compounds etc. are provided or employed in theblending, they may be employed in any suitable proportions or ratios,with the provision, as discussed hereinafter, that appropriate pHcontrol and solubility factors are taken into account. Preferredsequestering agents are polycarboxylic acids, such as tricarboxylicacids; polyaminopolycarboxylic acids (defined here as having more thanone amine group that is substituted by at least one carboxylic acid);and monoaminopolycarboxylic acids (defined here as having no more thanone amine group, the amine group substituted by more than one carboxylicacid); and the ammonium or potassium salts thereof. Particularlypreferred acids are citric acid, nitrilotriacetic acid (amonoaminopolycarboxylic acid), 2-hydroxyethyliminodiacetic acid (HEIDA,a monoaminopolycarboxylic acid), malic acid, tartaric acid, andN-(2-hydroxyethyl)ethylenediaminetriacetic acid (apolyaminopolycarboxylic acid). As indicated, the chelating agent will besupplied in an effective amount, i.e., an amount sufficient or effectiveto chelate aluminum or aluminum fluoride species which may becomeavailable during application of the invention. The chelating agent maythus be employed in varying amounts depending on whether or not it alsoperforms the function of supplying hydrogen ion, and, in such cases, thechelating agent will be supplied in an amount sufficient to achieve thedesired or required hydrogen ion concentration, which may be more thanthe amount needed for chelation. In one of the preferred aspects of theinvention, wherein ammonium bifluoride is blended in aqueous solutionand citric acid or N-(2-hydroxyethyl)ethylenediaminetriacetic acidprovides citrate or N-(2-hydroxyethyl)ethylenediaminetriacetate ion forsequestering or chelating aluminum ion or aluminum fluoride species, thecitric acid or N-(2-hydroxyethyl)ethylenediaminetriacetic acid may alsoprovide sufficient hydrogen ion for production of HF.

Sodium salts of the acids of the invention could also be used, takinginto account the lower solubility of NaF.

As indicated, low pH is an important aspect of the compositions andmethod of the invention. In general, the compositions should beformulated or blended to have a pH below 3, preferably 2 or below, mostpreferably below 1.5 or 1.0. Suitable addition or blending of anon-interfering acid, e.g., HCl, may be employed to insure the desiredlow pH levels. Generally, the sequestering agent should have asolubility of at least 1 percent, preferably 1 to 10 percent, at 25° C.and a pH of 3 or less.

As indicated, the boron source comprises a compound, or mixturesthereof, or a solution of the compound or compounds with an aqueousliquid, providing boron ions or boron-containing anions which arereactive with a component of the aqueous liquid to form BF₄ anion in theaqueous liquid. Suitable boron sources include boric acid, boronhalides, boron hydrides, and metal borates, such as alkali metalborates.

The compositions of the invention are particularly suited for acidizingsandstone formations containing high silt and clay content andHCl-sensitive minerals, such as zeolites and chlorites. As indicated,the matrix stimulation compositions of the invention may be used as (1)a preflush, (2) the main acid treatment, or (3) as a postflush.

The compositions of the invention remove formation damage caused by clayand other aluminosilicate minerals. They also minimize hydrated silicaprecipitation, as a result of the chelating component specified. Finesare also prevented from migrating as a result of the deposition of acoating of borosilicates and silicates, which fuse the fines to eachother and to the sand matrix of the formation. This coating is alsouseful during a preflush application to desensitize the HCl-sensitiveminerals by coating them and thereby protecting them from the adverseeffects of HCl.

In a typical treatment, a preflush such as toluene, xylene, or the likemay be employed, if desired, to clean the wellbore and surroundingformation of organic deposits such as paraffins or asphaltines.Optionally, the preflush to remove organic deposits may be followed by apreflush of HCl or an organic acid to dissolve carbonates in theformation. Where the formation is acid sensitive, i.e., susceptible toan initial decrease in permeability upon contact with HCl, fluoboricacid is beneficially employed as the preflush, as taught in U.S. Pat.No. 4,151,878, hereby incorporated by reference.

When any desired preflushes have been completed, a suitable volume ofthe composition of the invention is injected in a conventional manner asthe main acidizing composition at a matrix rate, i.e., at a rate whichdoes not fracture the formation.

In a situation wherein a composition of the invention is used for themain acidizing treatment, a preferred treatment sequence is as follows:

-   -   1. Circulation and establishment of infectivity with ammonium        chloride brine.    -   2. Injection of an aqueous solution containing 5 percent        ammonium chloride and 10 percent of a mutual solvent such as        ethylene glycol monobutylether.    -   3. Preflush with an aqueous solution containing 10 percent        glacial acetic acid.    -   4. Injection of the matrix stimulation fluid (the composition of        the invention), with an optional shut-in period.    -   5. Overflush or postflush with an aqueous solution containing 5        percent by weight ammonium chloride or 10 percent glacial acetic        acid.    -   6. Flowback.

EXAMPLE 1

To illustrate the preparation of a composition of the invention, and howthe concentration of HF may be fine tuned, the following procedure wasconducted. About 3 wt % hydrofluoric acid (HF, MW=20), about 10 wt %citric acid, and water are mixed together to form a pre-compositionblend or mixture. About 1.2 wt % boric acid (MW=61.8) is then blendedwith the mixture. Reaction of the boric acid with HF in the mixtureresults in the production of fluoboric acid, and reduces the free HFconcentration to about 1.5 wt % in the final mixture. As will beappreciated, in order to remove 1.5 wt % (0.75 molar) HF from a 3 wt %(1.5 molar) HF solution by reaction with boric acid to form fluoboricacid (MW=87.8), a concentration of 0.188 molar boric acid is required inthe mixture. That is, for every 4 moles of HF, 1 mole of boric acid(H₃BO₃) is required in the reaction mixture to produce 1 mole offluoboric acid (eq. 4). Therefore, about 1.16 wt % of boric acid in amixture with about 3 wt % HF, will leave about 1.5 wt % free HF acid inthe composition. Similarly, if only about 1 wt % free HF is desired whenstarting with about 3 wt % HF, about 2 wt % HF needs to be reacted withthe boric acid. About 1.54 wt % of boric acid in a mixture with about 3wt % HF solution would leave about 1 wt % free HF in the composition.When using a composition of the present invention comprising about 1.5wt % free HF and fluoboric acid, the formation reaction capacity of thissystem is dramatically increased over that of a conventional 1.5% HFsolution, because as free HF is spent on the formation, additional HFcan be generated in the formation through hydrolysis of the fluoboricacid. Advantageously, however, the formation never “sees” excessive HFacid concentration.

As mentioned, compositions of the present invention can be formulated athigh acid concentration and diluted on site during the treatmentimplementation. Such dilution of the composition can permit thetreatment to be pumped at higher rates (e.g., 6-10 BPM). The overallvolume of the treatment can be reduced due to the ability to add thehydrofluoric acid to the formulation at higher concentration (e.g. inthe form of fluoboric acid), without damaging the formation.

In the following additional examples, exemplary compositions accordingto the invention are referred to as New Acid and New Acid II. Unlessexpressed otherwise or evident from the context, all percentages ofcomponents of mixtures expressed hereinafter are by weight, based on thetotal weight of the mixture including the component.

New Acid was prepared by mixing about 13.4 percent citric acid, about9.8 percent ammonium bifluoride, and about 4.9 percent boric acid infresh water. The ammonium bifluoride and the citric acid react (reaction2) to form hydrogen fluoride in solution, which reacts with the boricacid. There is thus formed an aqueous solution containing various ionicspecies, including fluoborate ion and citrate ion.

New Acid II was prepared by mixing about 13.4 percentN-(2-hydroxyethyl)ethylenediaminetriacetic acid, about 9.8 percentammonium bifluoride, and about 4.9 percent boric acid in fresh water.The ammonium bifluoride and the HEDTA react to form hydrogen fluoride insolution, which reacts with the boric acid. There is thus formed anaqueous solution containing various ionic species including fluoborateion and N-(2-hydroxyethyl)ethylenediaminetriacetate ion.

EXAMPLE 2

Five samples from a sandstone subterranean formation in South Americacontaining 58% quartz, 12% plagioclase, 4% calcite, 20% chlorite, and 6%smectite were prepared. The samples were then individually contactedwith aqueous solutions comprising or formulated, as follows: 12/3HCl/HF, 9/1 HCl/HF, 3/1 HCl/HF, fluoboric acid, and New Acid. Thestarting ingredients of the acid compositions are shown in Table 1.

TABLE 1 New Fluoboric 12/3 mud 9/1 mud 3/1 mud Acid acid acid acid acidWater (ml) 97.0 69.9 37.5 76.0 90.7 37% HCl (ml) 0.0 18.2 62.4 23.9 9.2Ammonium bifluoride 9.8 12.0 4.8 1.5 1.5 (NH₄HF₂) (g) Boric acid(H₃BO₃)(g) 4.9 6.0 0.0 0.0 0.0 Citric Acid (C₆H₈O₇) 13.4 0.0 0.0 0.0 0.0(g)

Procedure

For each acid solution to be tested, a sequential spending test wasperformed to simulate the process of the acid penetrating into asubterranean formation and reacting with formation rock that had beenpreflushed with 10% citric acid. The testing procedure is depicted inFIG. 1. Initially, a large portion of rock was contacted with 10% citricacid preflush for 1 hour. This rock was separated from the preflush, andportions were treated individually, as follows. (These samplescollectively were representative of a formation that had been treatedwith 10% citric acid, and they were then used to simulate regions of theformation, successively farther away from the wellbore, that weretreated with the acid being tested.) A 50 ml sample of a fresh solutionof the acid being tested was loaded into a plastic bottle and heated toabout 71° C. Five grams of the 10% citric acid treated sample was addedto the bottle and allowed to react with the acid for 1 hour. The spentacid in this first batch (batch #1) was filtered out, and a 5 mlfiltrate sample was extracted for ion analysis. This first batchrepresented a region of the formation nearest the wellbore (designatedRegion 1 in FIG. 1). A second sample of 4.5 grams of the 10% citric acidtreated sample was added to the remaining 45 ml of filtrate of the acidbeing tested. This batch was designated as batch #2 and simulated spentacid from Region 1 penetrating further into the subterranean formationand contacting a portion of the formation (Region 2 in FIG. 1) that hadnot yet been contacted with the acid being tested. After one hour, thespent acid from batch #2 was filtered out, and a 5 ml filtrate samplewas extracted for ion analysis. A third batch of 4.0 grams of the 10%citric acid treated sample was added to the remaining 40 ml of filtrate.This batch was designated as batch #3. It was then handled as theprevious batches had been handled, and finally a similar procedure wasused for batch #4 made from 3.5 grams of the 10% citric acid treatedsample and 35 ml of the filtrate from batch #3.

FIG. 2 indicates that as the mud acids penetrate into a formation, spentacid comes into contact with fresh subterranean formation minerals, andthe concentration of silicon ions is reduced. The fluoboric acid'scapability of slowly generating HF to continue dissolve aluminosilicateminerals renders a flat profile as dissolution and precipitationprocesses reach equilibrium. The New Acid system results indicatedramatically improved Si ion concentrations away from the near wellborearea.

FIG. 3 shows the fluoride concentrations in the spent acid solutions ofthe 12/3 HCl/HF, 9/1 HCl/HF, 3/1 HCl/HF, fluoboric acid, and New Acid.The mud acids generated fluoride rapidly, so the fluoride concentrationreached a plateau. However, the New Acid generated fluoride ions slowly,since they are generated principally by the reaction between the HFgenerated from the fluoborate species in the solution and the mineralsin the sample. The silicon ion and fluoride concentrations from therespective batches are shown in Table 2.

TABLE 2 Batch #1 Batch #2 Batch #3 Batch #4 Silicon Fluoride SiliconFluoride Silicon Fluoride Silicon Fluoride New Acid 0.11 1.3 0.22 2.30.31 2.7 0.27 2.5 Fluoboric acid 0.17 1.6 0.17 1.6 0.13 1.5 0.18 1.812/3 mud acid 0.27 1.2 0.21 1.3 0.13 1.1 0.046 0.60 9/1 mud acid 0.0880.42 0.049 0.32 0.023 0.31 0.0087 0.33 3/1 mud acid 0.083 0.46 0.0520.44 0.047 0.38 0.036 0.34

EXAMPLE 3

A synthetic blend of 90% 100 mesh silica sand with 10% zeolite was usedto simulate a subterranean formation containing HCl minerals, e.g.zeolite, typically found in the Gulf of Mexico. Samples were contactedindividually with solutions, as follows: 9/1 HCl/HF, 3/1 HCl/HF,fluoboric acid, and New acid. The test procedure followed was the sameas that of Example 1. The results of the tests are shown in Table 3.

TABLE 3 Batch #1 Batch #2 Batch #3 Batch #4 Silicon Fluoride SiliconFluoride Silicon Fluoride Silicon Fluoride New Acid 0.070 1.0 0.15 1.60.27 2.6 0.37 2.9 Fluoboric acid 0.095 0.99 0.14 1.4 0.19 1.7 0.19 1.712/3 mud acid 0.27 1.2 0.21 1.3 0.13 1.1 0.046 0.60 9/1 mud acid 0.0880.47 0.11 0.45 0.11 0.46 0.11 0.45 3/1 mud acid 0.066 0.31 0.10 0.480.12 0.48 0.074 0.32

FIG. 4 shows graphically that the concentrations of Si did not increasesubstantially from Batch #1 through Batch #4 utilizing mud acids.

However the New Acid continued to dissolve minerals and maintain thesilicon ions in solution, and the silicon concentration in solutionaccumulated from samples extracted from Batch #1 through Batch #4.

FIG. 5 shows that fluoride was continuously generated as the New Acidsequentially contacted the samples and the fluoride concentrationtherefore rose. On the other hand, the fluoride concentrations reachedequilibrium rapidly with the mud acids, and the mud acids were unable tocontinue to dissolve minerals.

EXAMPLE 4

In this example, a sandstone core containing 2% kaolinite clay was used.The core was sensitized by injection of 6% NaCl solution, then damagedby fresh water (FIG. 6). The core was then acidized by the followingprocedure. First, a 3% NH₄Cl solution was injected through the core aspreflush, followed by 15% HCl. The core was then treated with New Acid.No apparent increase of permeability was observed during the acidinjection. The core was then shut in for 16 hours at about 90° C. Thepermeability was then measured again using 6% NaCl. A significantrecovery of permeability was achieved. The 6% NaCl solution was againinjected through the core to again sensitize the core. However, thoughfresh water was injected following the 6% NaCl in an attempt to againdamage the core, no damage occurred, showing that the New Acid treatmentsuccessfully stabilized the core.

EXAMPLE 5

Long term spending tests were performed to investigate whether or notreaction byproducts which cause precipitants will potentially plug asandstone matrix. Batches of solution and minerals were placed in awater bath set at about 81° C. temperature for 24 hours, and weight losswas measured as a function of time. If a batch showed weight gain duringreaction, it was an indication that precipitation had occurred.

Weight loss measurements were also conducted to investigate the effectof acid solution volume to mineral mass ratio on reactions. The solidcomposition employed was a blend comprising 10% zeolite and 90% 100 meshsilica sand. The acid solutions used included New Acid, fluoboric acid,9/1 HCl/HF mud acid, and 3/1 HCl/HF mud acid. The results are shown inTables 4, 5, and 6.

Table 4

Wt. Loss % from initial 2.5 g of minerals (90% Silica sand+10% zeolite)reacting with 25 ml of acid

TABLE 4 Wt. Loss % from Initial 2.5 g of minerals (90% Silica sand + 10%zeolite) reacting with 25 ml of acid Time (hr) New Acid 9/1 mud acid 3/1mud acid Fluoboric acid 1 5.5 5.3 4.9 3.8 3 7.9 6.5 6.3 6.3 5 8.7 7.26.9 7.0 7 9.7 8.1 7.5 8.1 22-23 11.0 9.2 6.9 7.5 24  12.1 8.8 7.0 8.3

Table 5

Wt. Loss % from initial 5 g. of minerals (90% Silica sand+10% zeolite)reacting with 25 ml of acid

TABLE 5 Wt. Loss % from Initial 5 g. of minerals (90% Silica sand + 10%zeolite) reacting with 25 ml of acid Time (hr) New Acid 9/1 mud acid 3/1mud acid Fluoboric acid 1 3.5 4.5 4.0 3.5 3 4.9 4.5 5.0 3.9 5 5.8 4.94.7 4.7 7 6.3 4.9 5.1 5.7 22-23 9.4 4.7 5.2 6.4 24  10.1 4.7 5.5 6.8

Table 6

Wt. Loss % from initial 10 g. of minerals (90% Silica sand+10% zeolite)reacting with 25 ml of acid

TABLE 6 Wt. Loss % from Initial 10 g. of minerals (90% Silica sand + 10%zeolite) reacting with 25 ml of acid Time (hr) New Acid 9/1 mud acid 3/1mud acid fluoboric acid 1 2.2 2.8 3.2 2.5 3 3.2 2.7 3.3 2.9 5 3.3 2.73.2 2.3 7 3.3 3.1 2.8 1.9 22-23 5.0 3.1 3.2 3.5 24  4.9 3.3 2.9 4.0

Mud acids (9/1 and 3/1 HCl/HF) were rapidly spent, and becamenon-reactive shortly after acid-mineral contact. Potential weight gaindue to precipitation was also observed. Fluoboric acid did slowlygenerate HF to sustain mineral dissolution. However, it causedprecipitation by gaining weight during the reaction process. The NewAcid continuously dissolved minerals, even at a low acid/mineral ratio(25 ml acid/2.5 g mineral (FIG. 7), no weight gain was seen, theprecipitation potential was therefore minimal. FIGS. 7 through 9 showgraphically the results of reacting 25 ml of acid with 2.5, 5, and 10grams of solids. The results demonstrate that a chelating agent, citricacid, in the New Acid system was essential in preventing precipitation.

EXAMPLE 6

The long term weight loss of glass slides were measured by reactingvarious acid formulations with silica glass to determine the dissolvingpower of the acids. The tests were conducted at about 90° C. The acidsolutions used in this series of tests include New Acid, fluoboric acid,12/3, 9/1, 3/1 mud acids, and New Acid II. New Acid II was formulated byblending 13.4% N-(2-hydroxyethyl)ethylenediaminetriacetic acid, 9.8%ammonium bifluoride (NH₄HF₂), and 4.9% boric acid (H₃BO₃) in water. Twohundred fifty milliliters samples of each acid solution wererespectively allowed to contact 4.5 gram silica glass slides for 48hours. Each slide was then dried and weighed. The results of thisprocedure are shown in Table 7.

Table 1

TABLE 7 12/3 mud New Flouboric 9/1 mud 3/1 mud New Acid Time (hr) acidAcid acid acid acid II 0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 19.0 6.8 1.6 5.71.0 2.4 3 47.1 25.2 5.1 11.6 8.4 9.6 4 51.8 29.5 7.3 13.5 10.5 12.1 564.2 33.0 9.6 17.0 13.5 14.9 6 78.9 12.3 20.9 17.5 7 85.2 13.9 23.6 19.522 95.8 73.4 35.7 42.5 36.3 35.7 23 100.0 74.5 35.9 43.6 36.6 37.0 2880.1 40.3 45.1 39.1 40.9 29 80.6 40.3 45.2 39.1 41.7 30 40.8 45.5 39.342.4 45 97.3 46.0 47.0 41.0 51.9 46 46.0 47.2 41.4 52.2 47 48.4 47.341.7 52.6 48 48.4 47.9 41.7 52.8 50 49.4 48.3 42.2

Table 7 shows that mud acids react rapidly with silica and then lose thestrength needed to continue the reaction. The fluoboric acid continuedto dissolve the silica throughout the 48 hour time period. The New Acidand the New Acid II exhibited sustained reaction power. Because theHEDTA in New Acid II is a higher molecular weight (M.W. 278) material,equivalent weight percent generated less molar of HF than the citricacid (M.W. 192) based New Acid. Accordingly, the reaction capability wasdiminished. The 13.4% citric acid generated about 2.2% HF, which isabout 1.1 molar, upon initial mixing of the starting ingredients with afinal generated amount of about 3.5% HF. The 13.4% HEDTA generated onlyabout 0.75 molar of HF upon mixing of initial ingredients with a finalgenerated amount of HF of about 2.5%.

EXAMPLE 7 (MAIN FLUID)

A subterranean formation with permeability from 60 to 500 md, containing60% quartz, 9% dolomite, 10% zeolite, 5% smectite, 1% chlorite, 10%feldspar, and 5% albite is to be acidized to remove damage caused byclay swelling and dispersion. New Acid may be applied in the followingmanner to restore permeability. A preflush of 5% NH₄Cl in a volume of 40gallons per ft. of the subterranean formation penetrated by the wellboreis applied, followed by 5% acetic acid in a volume of 50 gallons per ft.of the subterranean formation penetrated by the well bore. The preflushis followed by the main treating fluid, New Acid, prepared by mixing 13%citric acid or HEDTA, 10% NH₄HF₂, and 2 to 5% H₃BO₃ water in a volume of100 gallons per ft. of the formation. The main treating acid is followedby 5% acetic acid, in a volume of 50 gallons per ft. of the formationpenetrated by the well bore, and finally displaced by 5% NH₄Cl in avolume of 50 gallons per ft. of the formation penetrated by the wellbore. Common additives, such as corrosion inhibitors, non-emulsifyingagents, anti-sludging agents, and water wetting agents, may be used inthe acetic acid and main acid stages.

EXAMPLE 8 (USED AS A PREFLUSH)

A subterranean formation containing 80% quartz, 10% illite, 2% calcite,and 3% feldspar, and 5% kaolinite is to be treated. The subterraneanformation may be treated by a first preflush with 3% NH4Cl in a volumeof 50 gallons per ft. of subterranean formation penetrated by the wellbore. The formation may then be preflushed with New Acid (prepared bymixing 10% to 13% citric acid, 10% NH₄HF₂, and 2 to 5% H₃BO₃ in water)in a volume of 50 gallons per ft. of the subterranean formationpenetrated by the well bore. The formation may then be treated with a9/1 HCl/HF mud acid in a volume of 100 gallons per ft. of thesubterranean formation penetrated by the well bore, followed by 15% HClin a volume of 50 gallons per ft. of the formation penetrated by thewell bore. Finally the HCl may be displaced by 3% NH₄Cl in a volume of50 gallons per ft. of the formation penetrated by the well bore.

EXAMPLE 9

In this example, a sandstone core containing trace to 5% zeolite wasused. The core was saturated by injection of Ewing Bank Formation brinefrom the Gulf of Mexico. The core was then acidized by injecting first a5% NH₄Cl solution as preflush, followed by a second day re-injection ofthe 5% NH₄Cl solution. The core was then treated with an acid solutioncomprising 10% citric acid and 1.5% hydrofluoric acid. The core flowprocedure was, as follows:

-   -   (1) The unconsolidated sand from the formation was packed into a        1″ in diameter by 12″ in length core holder, and the core was        heated to about 61° C.    -   (2) A 300 ml quantity of Ewing Bank formation brine was injected        through the core at 10 ml/min.    -   (3) The brine injection was followed by injection of 300 ml of        5% NH₄Cl through the core, also at 10 ml/min.    -   (4) The acid solution to be tested was injected at 300 ml of        acid through the core at 10 ml/min.    -   (5) Test acid solution injection was followed by injection of        300 ml of 5% NH₄Cl through the core at 10 ml/min.    -   (6) Finally, 300 ml of fresh water was injected through the core        at 10 ml/min.

The differential pressure across the core was measured through allstages of fluid injection so the change of permeability could bemonitored. When the test acid solution was injected through the core,the test solution yielded a flat but slightly downward trend inpermeability response. The final permeability measured by 5% NH₄Cl andfresh water showed the core had lower permeability than it initiallyhad.

No apparent increase of permeability was observed during the citric-HFacid injection. The permeability was then measured again upon injectionof 5% NH₄Cl solution. A recovery of permeability was achieved. The 5%NH₄Cl solution was again injected to sensitize the core. However, thoughtap water was injected following the injection of 5% NH₄Cl solution inan attempt to again damage the core, no damage occurred showing thatcitric-HF acid successfully stabilized the core. The results are showngraphically in FIG. 11.

EXAMPLE 10

The procedure of Example 9 was repeated, except that the acid solutionto be tested contained 13.4% citric acid, 10% ammonium bifluoride, and5% boric acid. When the test acid solution was injected through thecore, the acid yielded a continuous downward trend in permeabilityresponse. The final permeability measured by 5% NH4Cl and fresh watershowed the core had much lower permeability than it initially had. Theresults are shown graphically in FIG. 12.

EXAMPLE 11

The procedure of Example 8 was repeated, except that the acid solutioncomprised 10% citric acid, 5% boric acid, 1.5% hydrofluoric acid and 5%NH₄Cl. No significant increase of permeability was observed during theacid solution injection. The permeability was then measured again uponinjecting 5% NH₄Cl solution. A significant recovery of permeability wasachieved. The 5% NH₄Cl solution was again injected to sensitize thecore. However, though tap water was injected following the injection of5% NH₄Cl solution in an attempt to again damage the core, no damageoccurred. The results are shown graphically in FIG. 13.

EXAMPLE 12

The procedure of Example 10 was repeated, except that the acid solutionto be tested contained 10% citric acid, 1% HF and 5% boric acid. Whenthe test acid solution was injected through the core, the acid yieldedan increasing permeability trend as the injection progressed. The finalpermeability measured by 5% NH4Cl and fresh water showed the core hadlower permeability than it initially had. The results are showngraphically in FIG. 14.

EXAMPLE 13

The procedure of Example 10 was repeated, except that the acid solutionto be tested contained 10% citric acid, 1.5% HF, 0.6% boric acid, and 5%NH₄Cl. When the test acid solution was injected through the core, theacid yielded an increasing permeability trend as the injectionprogressed. The final permeability measured by 5% NH4Cl and fresh watershowed the core permeability was equal to its initial value. The resultsare shown graphically in FIG. 15.

While the compositions of the invention have been described generally inrelation to the treatment of subterranean formations for permeability,they may also be employed for increasing the permeability of a proppantpack, present in a fracture in a subterranean formation, whosepermeability has been reduced by fines, particles, or bridging.Concentrations and conditions of application in such a procedure areidentical or analogous to those employed in treating a subterraneanformation. For example, the concentrations of fluoboric acid or BF₄ ⁻anion will be effective to increase the permeability of the proppantpack.

1. A composition for treating a subterranean formation comprising anaqueous acidic mixture having pH of less than 2 formed by blending anaqueous liquid; an amount of fluoboric acid sufficient to provide from 1to 20 percent BF₄ ⁻ anion; and an effective amount of an acid, ormixture of acids, which chelate aluminum ions and aluminum fluoridespecies, selected from the group consisting of citric acid, malic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, an ammonium orpotassium salt or salts of such acids, or a mixture thereof.
 2. Thecomposition of claim 1 in which non-interfering ionic species arepresent in an amount effective to provide an ionic strength for theaqueous mixture sufficient to inhibit migration of clay particles.
 3. Acomposition for treating a subterranean formation comprising an aqueousacidic mixture having a pH of less than 2 formed by blending an aqueousliquid; amounts of a fluoride ion source and a boron source sufficientto provide a concentration of BF₄ ⁻ anion effective to provide from 1 to20 percent BF₄ ⁻ anion; and an effective amount of an acid, or mixtureof acids, which chelate aluminum ions and aluminum fluoride species,selected from the group consisting of citric acid, malic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, or an ammonium orpotassium salt or salts of such acids, or a mixture thereof.
 4. Thecomposition of claim 3 in which non-interfering ionic species arepresent in an amount effective to provide an ionic strength for theaqueous mixture sufficient to inhibit migration of clay particles.
 5. Acomposition for treating a subterranean formation comprising an aqueousmixture formed by blending at a pH of less than 2 an aqueous liquid,amounts of HCl, a fluoride ion source, and a boron source sufficient toprovide a concentration of BF₄ ⁻ anion effective to increase thepermeability of the formation, and an effective amount of an acid, ormixture of acids, which chelate aluminum ions and aluminum fluoridespecies, selected from the group consisting of citric acid, malic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, an ammonium orpotassium salt or salt of such acids, or a mixture thereof.
 6. Thecomposition of claim 5 in which the aqueous mixture comprises from 1 to20 percent BF₄ ⁻ anion.
 7. The composition of claim 5 in whichnon-interfering ionic species are present in an amount effective toprovide an ionic strength for the aqueous mixture sufficient to inhibitmigration of clay particles.
 8. A method of treating a subterraneanformation to increase permeability comprising injecting into theformation an effective amount of an aqueous acidic mixture formed at apH of less than 2 by blending an aqueous liquid, an amount of fluoboricacid sufficient to increase the permeability of the formation, and aneffective amount of an acid, or mixture of acids, which chelate aluminumions and aluminum fluoride species, selected from the group consistingof citric acid, malic acid, N-(2-hydroxyethyl)ethylenediaminetriaceticacid, or an ammonium or potassium salt or salts of such acids, or amixture thereof.
 9. The method of claim 8 in which the aqueous mixturecomprises from 1 to 20 percent BF₄ ⁻ anion.
 10. The method of claim 8 inwhich non-interfering ionic species are present in an amount effectiveto provide an ionic strength for the aqueous mixture sufficient toinhibit migration of clay particles.
 11. A method of treating asubterranean formation to increase permeability comprising injectinginto the formation an effective amount of an aqueous acidic mixtureformed at a pH of less than 2 by blending an aqueous liquid, and amountsof a fluoride ion source and a boron source sufficient to provide aconcentration of BF₄ ⁻ anion effective to increase the permeability ofthe formation, and an effective amount of an acid, or mixture of acids,which chelate aluminum ions and aluminum fluoride species selected fromthe group consisting of citric acid, malic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, or an ammonium orpotassium salt or salts of such acids, or a mixture thereof.
 12. Themethod of claim 11 in which the aqueous mixture comprises from 1 to 20percent BF₄ ⁻ anion.
 13. The method of claim 11 in which non-interferingionic species are present in an amount effective to provide an ionicstrength for the aqueous mixture sufficient to inhibit migration of clayparticles.
 14. A method of treating a subterranean formation to increasepermeability comprising injecting into the formation an effective amountof an aqueous acidic mixture formed at a pH of less than 2 by blendingan aqueous liquid, amounts of HCl, a fluoride ion source, and a boronsource sufficient to provide a concentration of BF₄ ⁻ anion effective toincrease the permeability of the formation, and an effective amount ofan acid, or mixture of acids, which chelate aluminum ions and aluminumfluoride species, selected from the group consisting of citric acid,malic acid, N-(2-hydroxyethyl)ethylenediaminetriacetic acid, or anammonium or potassium salt or salts of such acids, or a mixture thereof.15. The method of claim 14 in which the aqueous mixture comprises from 1to 20 percent BF₄ ⁻ anion.
 16. The method of claim 14 in whichnon-interfering ionic species are present in an amount effective toprovide an ionic strength for the aqueous mixture sufficient to inhibitmigration of clay particles.
 17. A composition for treating a proppantpack in a fracture in a subterranean formation comprising an aqueousacidic mixture formed at a pH of less than 2 by blending an aqueousliquid; an amount of fluoboric acid sufficient to provide from 1 to 20percent BF₄ ⁻ anion; and an effective amount of an acid, or mixture ofacids, which chelate aluminum ions and aluminum fluoride species,selected from the group consisting of citric acid, malic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid, or an ammonium orpotassium salt or salts of such acids, or a mixture thereof.
 18. Amethod of treating a proppant pack in a fracture in a subterraneanformation to increase permeability of the proppant pack comprisinginjecting into the proppant pack an effective amount of an aqueousacidic mixture formed at a pH of less than 2 by blending an aqueousliquid, an amount of fluoboric acid sufficient to increase thepermeability of the proppant pack, and an effective amount of an acid,or mixture of acids, which chelate aluminum ions and aluminum fluoridespecies, selected from the group consisting of polycarboxylic acids,polyaminopolycarboxylic acids, an ammonium or potassium salt or salts ofsuch acids, and a mixture thereof.